[Updated at 9:24 a.m.] Negotiations were under way to free the hostages when gunfire erupted aboard the pirated vessel, a statement from U.S. Central Command said.

"As they responded to the gunfire, reaching and boarding the Quest, the forces discovered all four hostages had been shot by their captors. Despite immediate steps to provide life-saving care, all four hostages ultimately died of their wounds," the statement said.

A reaction force boarding the S/V Quest "was engaged by pirates," two of whom died during the confrontation. Thirteen other pirates were captured, along with two who were already in the custody of U.S. forces, according to central command.

Two other pirates were already dead when U.S. forces boarded the S/V Quest, the statement said. In all, 19 pirates are believed to have been involved in the hijacking.